1. Field of the Invention
The present invention relates to containers for use in detecting micro-organisms in samples of substances, which may consist either of substances intended for human consumption, such as foods, drinks or pharmaceuticals, or of pathological or other laboratory substances.
2. Description of the Prior Art
A number of known methods for detecting micro-organisms include the use of electrical cells, in which electrodes are in contact with a micro-biological sample in a growth medium. Such methods include measurement of sample impedance, measurement of dissolved oxygen concentration by applying a pulsed voltage to the growth medium/electrolyte and, in a third method, changes in potential differences between two electrodes immersed in a growth medium containing the micro-organisms are monitored. This latter method has several advantages over other methods of detecting micro-organisms and apparatus for carrying out this method is described and claimed in our copending British patent application No. 8317685 (publication No. 2142433) and International application No. PCT/GB84/00235 (publication No. WO85/00225)). Nevertheless, the present invention is applicable to any method of micro-organism detection requiring the use of a cell (or test container) in which one or more electrodes would have to be in contact with the growth medium.
A problem has arisen, particularly in the case of disposable test containers manufactured with growth medium in situ and which are expected to have a shelf life of up to, say, 18 months, before a microbological sample is introduced to the container for testing. During storage, the electrodes are in contact with the growth medium and hence are susceptible to corrosion. Corrosion of the electrode(s) gives raised concentrations of cations in the growth medium, which might inhibit growth of the micro-organisms to be tested, and hence lead to spurious test results.
An attempt has been made to mitigate the likelihood of electrode corrosion by providing test containers with electrode(s) in the lid, so that the container can be stored with the electrode(s) out of contact with the growth medium until the test is due to start, when the container is inverted to bring the sample into contact with the electrode(s). Nevertheless, it has been found that the electrode(s) may still show signs of corrosion because of being kept in contact with the vapour above the growth medium. The method referred to above, involving changes in potential difference between two passive electrodes, benefits from using electrodes of dissimilar metals, one being say aluminium or zinc and the other being a noble metal, such as gold or platinum, because there is a greater drop in potential difference once growth of the micro-organism has reached a particular concentration. The greater the change in potential difference, the less significant is the electrical noise in the system, so that the drop will be more readily apparent. As a result of the need for dissimilar metals, at least one of the electrodes is likely to be relatively prone to corrosion.